The enzyme oxidizes or reduces the hydroxy/keto group on C17 of estrogens and androgens in mammals and regulates the biological potency of these steroids. The mammalian enzyme is bifunctional and also catalyses EC 1.1.1.270, 3beta-hydroxysteroid 3-dehydrogenase [3]. The enzyme also acts on (S)-20-hydroxypregn-4-en-3-one and related compounds, oxidizing the (S)-20-group, but unlike EC 1.1.1.149, 20alpha-hydroxysteroid dehydrogenase, it is Si-specific with respect to NAD(P)+.